It's actually a bunch of filters over-driven into each other, so much so that the original phase synthesized waveform barely makes it through. The final filter, a low pass, is operated by a looping, low frequency oscillator of which the time of the attack phase can be varied. Also the cut point of that last filter can be adjusted. The sound is "played" mostly by changing the filter parameters, although principle notes can be fed into the filter chain for small variations.